i) Field of the Invention
The present invention relates to a stabilized antenna system including an antenna having fan beam directivity, that is, a wide beam width around a longitudinal axis of the antenna.
ii) Description of the Related Art
Conventionally, a directive antenna has been used for satellite communication on a ship or the like. The ship satellite communication was started by the MARISAT satellite, of the U.S.A., in 1976, which has been taken over and practiced by an international organization, INMARSAT, since 1982. For conducting such ship satellite communications, an antenna having a certain directivity is required.
For example, according to the technical requirements document for INMARSAT, as of June, 1987, a ship/earth station the G/T of the ship/earth station is provided with at least -4 dBK, and, in order to construct an antenna satisfying this requirement i.e., as a parabolic antenna, a diameter dimension of approximately 80 cm is demanded.
For ship satellite communication, a stabilized antenna system has been solely used. This stabilized antenna system is provided with a stabilization function in addition to a satellite tracking function.
That is, in order that an antenna mounted on a moving platform, in a ship or the like, can receive a radio wave sent from a satellite, it is necessary to track the satellite by driving the antenna. Such antenna driving and control functions can be constructed so as to carry out the stabilization of the antenna. For instance, the ship is inclined by waves on the sea, and by compensating for this inclination, good satellite tracking can be realized. The inclination parameter of the ship includes, for example, roll, pitch and the like. In order to stabilize the antenna against roll and pitch it is required to drive mechanically or electronically the antenna or its beam direction either sideways or lengthways. Hence, conventionally, a variety of techniques for driving the antenna have been developed.
In FIG. 29, there is shown a conventional stabilized antenna system, as disclosed in Japanese Patent Laid-Open No.Sho 51-115757. This antenna system is formed with a parabolic antenna 10 having pencil beam directivity, and a mount composed of members 12 to 16 for supporting the parabolic antenna 10.
By this mount, the parabolic antenna 10 can be angularly moved around an axis 12, around another axis 14 and also around a further axis 16 at the same time. Since the axis 16 is vertical, by angularly moving the parabolic antenna 10 around the axis 16, an azimuth the parabolic antenna 10 directs to can be controlled. Hence, this axis 16 is usually called an azimuth (AZ) axis.
In this conventional stabilized antenna system, an attitude sensor 18 is arranged on the axis 16 so as to rotate therewith. The attitude sensor 18 detects inclinations around the axes 12 and 14. By applying this detected result to the drive controls of the axes 12 and 14, while the inclinations are compensated for or stabilized, the satellite tracking by the parabolic antenna 10 can be properly performed.
As described above, all of three axes can be formed by mechanical axes. However, in this case, the structural designing becomes complicated, and thus the entire antenna system is apt to be high cost. In order to solve this problem, the axis structure is improved so as to be sufficient with two mechanical axes.
A a two-axis mechanical axis antenna system, for instance, is disclosed in "Development of a Compact Antenna System for INMARSAT Standard-B SEs in Maritime Satellite Communication", Shiokawa et al., Institute of Electronics and Communication Engineers of Japan, SANE 84-19, pp 17-24. In this antenna system, a short backfire antenna of 40 cm.phi., having a beam width of .+-.15.degree. is used.
On the basis of this structure, a stabilized antenna system can be implemented by a relatively simple mechanical structure.
However, in such a structure, a singular point is caused. The singular point, for instance, appears in the zenith direction, and, when the antenna faces in this direction under the inclined condition, a tracking error is caused. In order to deal with the singular point properly, a light and solid material is used for antenna and support frame construction to reduce a load of a drive motor. Alternatively, a relatively high performance AC servo motor is adopted and accordingly a high performance AC servo control circuit is used to drive the antenna by a high performance servo system. Furthermore, by improving the software, the tracking error near the singular point can be reduced.
However, these countermeasures require a particular material, expensive circuit adoption and the like, and increased cost of the antenna system can not be avoided. Furthermore, even when these countermeasures are applied, a tracking error of approximately 10.degree. is reported at the singular point.
In order to solve such problems, it is effective to use electronic beam steering for any of the axes. The electronic axis can be implemented by a phased array antenna.
The phased array antenna, for example, is formed by arranging a plurality of antenna elements as electrodes in a square lattice formed on an antenna plane. Furthermore, a phase shifter is provided for each antenna element, and by controlling the amount of phase shift of a signal for each antenna element, the beam direction of the antenna can be controlled. Also, as disclosed in Japanese Patent Application No. Hei 2-339317 proposed by the present applicant, by providing a phase shifter for each column of antenna elements arranged in a matrix form, the electronic axis can be implemented by a relatively simple construction.
As described above, by using two mechanical axes and one electronic axis, the singular point can be avoided and the stabilization can be carried out by a relatively simple and inexpensive construction. However, in this stabilization, a two to three axes control is required.
In general, the inclination of a ship is exhibited as a coordinate transformation, as shown in FIG. 30, wherein a coordinate system X(0)Y(0)Z(0) is represented by X(0) in the bow direction, Z(0) in the zenith direction when the ship is not inclined.
In this case, when a pitch occurs, the coordinate system is moved to X(1)Y(1)Z(1).
In turn, when a roll happens, the coordinate system is moved to X(2)Y(2)Z(2).
In FIG. 30, an angle v representing the inclination of the ship can be resolved into a component q1 around the elevation (EL) and a component q2 around the cross elevation (XEL) perpendicular to the EL axis. Each component q1 or q2 can be obtained by a matrix operation on the basis of the roll r or the pitch p.
For instance, when the EL and XEL axes are constructed as the mechanical and electronic axes respectively, the controls of the EL and XEL axes are carried out on the basis of the respective components q1 and q2.
However, this controlling becomes complicated with respect to carrying out the matrix operation. Hence, if the matrix operation can be omitted or eliminated, the construction of the antenna system can be simplified, and an inexpensive stabilized antenna system can be realized. For simplifying the construction and reducing the cost, an antenna system having a fan beam directivity is proposed.
In FIG. 31, there is shown another conventional stabilized antenna system using an array antenna having fan beam directivity. In the stabilized antenna system, as shown in FIG. 31, the array antenna 22 includes four antenna elements 20 aligned longitudinally. The array antenna 22 possesses fan beam directivity, as hereinafter described in detail, and is supported by an EL axis 24 so that the antenna elements may be arranged around the EL axis 24.
The EL axis 24 is rotatably supported by a U-shaped AZ axis frame 26. A gear 28 is mounted to one end of the EL axis 24, and an EL axis motor 30 is mounted to the AZ axis frame 26. A belt 32 is suspended between the gear 28 and the EL axis motor 30. Accordingly, by driving the EL axis motor 30, the EL axis 24 is rotated to turn the array antenna around the EL axis 24.
An AZ axis 34 is integrally secured to the AZ axis frame 26 on its central position and is rotatably held by a pedestal 36 having a T-shaped cross section, and a gear 38 is attached to the lower end of the AZ axis 34. An AZ axis motor 40 is mounted to the pedestal 36, and a belt 42 is extended between the gear 38 and the AZ axis motor 40. Hence, by driving the AZ axis motor 40, the AZ axis 34 is rotated to turn the array antenna 22 around the AZ axis 34.
The pedestal 36 eccentrically supports the AZ axis frame 26, the EL axis 24, the array antenna 22 and the like. That is, the pedestal 36 is mounted on a radome base 44 in an eccentric position from the center of the radome base 44. An access hutch 48 having sufficient size for operation is provided to the radome base 44 through a hinge 46 so as to be openable. The access hutch 48 is formed for an operator to insert his hand through the opened access hutch 48 for carrying out maintenance and inspection of the array antenna 22, it peripheral circuits and the like. As a result, the maintainability of the antenna system can be secured.
The radome base 44 constitutes the bottom part of a radome 50. The radome 50 for protecting the components of the antenna system from rainfall or the like is made of a material such as FRP or the like through which the radio wave can pass.
In FIGS. 32 and 33, there are shown antenna patterns of the array antenna 22 around the virtual XEL axis and the EL axis 24, respectively. The virtual XEL axis is a virtual axis perpendicular to the EL axis 24 and is not actually present in the antenna system shown in FIG. 31.
As apparent from FIGS. 32 and 33, the directivity of the array antenna 22 is wide around the virtual XEL axis and narrow around the EL axis 24. This property is generally called fan beam directivity. By using the fan beam directivity around virtual XEL axis, the stabilization of the component q2 is not required.
However, even in this case using the array antenna having the fan beam directivity, it is necessary to obtain the matrix operation of the component q1 around the EL axis 24, and the calculation for the control is still complicated.